First, you must establish a metadata link between your IdP and the SAMLtest SP by using the upload form and trusting the SAMLtest SP however your product does so.
The SAMLtest SP will test your IdP by issuing an AuthnRequest to it to see whether a user can login successfully and return to SAMLtest with assertion in hand. Please note that this test will fail to find your IdP if it is wrapped in an <EntitiesDescriptor> tag.
To begin the test, just input your entityID from your metadata in the box below as shown and SAMLtest will create an AuthnRequest and send your browser on its way. Your metadata will determine which binding is used, but it will typically be an HTTP-Redirect.